High-voltage transformer



10, 1946- H. HARTMANNY 2,407,420

HIGH VOLTAGE TRANSFORMER Filed Sept. 9, 1943 Patented Sept-1Q, 1943 HIGH-VOLTAGE TRANSFORMER Hans Hartmann, Baden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden,

Switzerland Application September 9, 1943, Serial No. 501,696 In Switzerland August 4, 1942 '7 Claims. 1

In plants where very high voltages occur it is preferable to use voltage instrument transformers which are connected between a single phase and earth. Several suitable arrangements for this purpose are known Where the voltage of the high voltage Winding increases steadily from Zero up to the high voltage potential. If the active part of such a transformer is located in an insulating casing containing a liquid or gaesous insulating medium, the surface of this part has a potential distribution ontrolled by the winding.

It is also known that inside such a transformer the potential gradient between the windings can be taken up by a U-shaped insulating cover which encloses the iron yoke on the high voltage side and is open on the earth side, whereby the potential increases continuousl from the open ends of the cover up to the high voltage end of the high voltage winding. This insulating cover can be located on a conductive support, for instance on the iron core which is also U-shaped. With such transformers it is also known that it is an advantage if the windings lie in close contact with the insulating material. Furthermore, it is also known to cover the low voltage inner side of th insulating cover, particularly over the section which encloses the bent yoke and is free of windings, with a metal layer.

With these known constructions of high voltage instrument transformers it is, however, not yet possible to make the best possible use of the high break-down value Of the solid insulation between the low and high Voltage. If there are oil gaps between both windings or between one winding and the insulating cover, the insulating strength of these gaps is much lower than that of the paper insulation; nevertheless these oil gaps are subjected to a higher field strength than the paper insulation. Furthermore the insulation problem is also considerably complicated by the instability of the electrical field caused by the winding edge at the high voltage end of the high voltage winding cylinder. Because even if the oil gaps are avoided due to the windings being in close contact with the insulating cover corona and leakage discharges occur at this edge which are very difiicult to control and cannot be prevented by merely providing the inside of the insulatin cover with a metal layer. The usual arrangement of rounded protective rings on the winding edges can increase the initial voltage at which corona occurs but does not, however, prevent dangerous leakage discharges. The distance and the insulation strength between both voltage carrying parts could therefore up to the present not be fixed with regard to the allowable breakdown voltage but was determined by the leakage discharge voltage and this therefore necessitated a much stronger insulation.

The difficulties described in connection with a, transformer with a closed U-shaped core and a U-shaped insulation cover closely fitted to the high and low voltage winding are avoided according to the present invention by every kind of edge effect being prevented by means of a controlled potential distribution, so that it is possible to dimension the insulation solely out of consideration for its breakdown strength.

constructional examples of the invention. are illustrated in the acompanying drawing in which Fig. 1 is a vertical sectional view of a high voltage transformer embodying the invention; and

Fig. 2 is a fragmentary vertical section through an upper portion of the core and insulating covering of a modification.

In Fig. 1 of the drawing, the U-shaped iron core a is encased in a correspondingly U-shaped sleeve of insulation a and is closed by the yoke 12. On both core limbs there is the cylindrical low voltage winding 0. The insulating cover d is a winding of fabric or paper type such as employed in cable constructions, being wound directly upon the winding 0 and extending without interruption over the yoke arch of the iron core, the insulating cover being open only at both the ends which abut against the lower yoke b.

The iron core and also the beginning of the high voltage winding e at the lower end are at earth potential. The winding e is a cylindrical winding composed of disc coils, the potential of this winding increasing from the bottom to the top where the highest potential prevails, this being connected to the high voltage terminal 1. In order to control the potential at the bend of the insulating cover where there is no winding and in order to avoid edge effects the insulating tube 11 is provided not only on the inside but also on the outside of this bend with conductive layers g, h, the outer layer g being maintained at high voltage potential by means of a conductive connection whilst the inner layer It is at earth potential. The transformer rests on a base plate is and is enclosed in a casing of ceramic insulating material 2'. The hollow spac inside this casing can be filled with insulating liquid or gas.

The conductive layer it can of course cover the entire inner surface of the insulating cover 07.. The layers can consist of metal or some conductive material, for instance the layers 9', h of Fig. 2 comprise varnish, mixed with metallic powder. They can for instance be made in such a manner that at least for the bent part of the cover it a conductive band is employed for the innermost and outermost layer, this band being either a metal band, a metallized band or a band which is impregnated or painted with a conductive material. The layers can also be wound on or into the insulating cover 12 or pressed on, sprayed or painted on to the cover. The layers g, h can also be made so as to allow liquid or gas to pass through, for instance in the form of wire netting and the like. It is furthermore also possible to make the layer on the outside (high voltage side) in the form of a bare or covered winding which in the latter case also serves as the first turn of the high voltage winding.

With the arrangement according to theinvention all parts of the insulating cover are stressed perpendicularly to the wall and solely as regards breakdown. The U-shaped form of the iron core and cable-like insulating cover d surrounding the core enables the potential distribution along its surface to be arranged in such a manner that the voltage increases from zero value at the lower open ends of the cover to a maximum value at the closed upper end. Since the outer surface of the insulating cover d has high voltage potential at its upper end, the core a which is at earth potential is completely screened from the upper metal cap which closes the insulating casing z and carries the high voltage terminal f.

A further advantage of the arrangement described is that due to the short distance between the windings the short-circuit voltage becomes so small that even with an iron core having a comparatively small cross-sectional area a very large measuring power can be obtained. At the same time the radial dimensions become reduced despite the large degree of safety which is obtained as regards the voltage.

I claim:

1. A high voltage transformer comprising a grounded U-shaped core closed by a yoke, a low voltage secondary winding including sections wound upon the legs of said core, a tubular insulating cover closely fitting said secondary sections and the portion of the core between the same, a high voltage primary winding on the legs of said core and closely fitting said insulating cover, said primary winding comprising a plurality of serially connected disc coils, with a low voltage terminal adjacent an end of a leg of the core and a high voltage terminal adjacent the bend of the U-shaped core, and means suppressing edge eliects at the high voltage ends of said windings, said means comprising conducting layers at and closely adjacent the portion of the insulating cover enclosing the bend of the core and not covered by the primary.

2. A high voltage transformer as recited in claim 1 wherein the outer conducting layer is electrically connected to the adjacent high potential end of the primary winding and the inner conducting layer is grounded.

3. A high voltage transformer as recited in claim 1 wherein said conducting layers are metallic.

4. A high voltage transformer as recited in claim 1 wherein said conducting layers comprise varnish containing conductive particles on the inner and outer surfaces of said insulating cover.

5. A high voltage transformer as recited in claim 1 wherein said conducting layers are fluidpermeable layers of conductive material at the inner and outer surfaces of said insulating cover.

6. A high voltage transformer as recited in claim 1 wherein said potential gradient means comprises a conductive winding at a surface of the 'bend portion of said tubular insulating cover and connected electrically to the adjacent terminals of the primary transformer windings on the respective legs of the core.

'7. A high voltage transformer as recited in claim 1 wherein said outer conducting layer comprises a wire wound upon the bend portion of said tubular insulating cover and connected electrically to the adjacent terminals of the primary windings.

HANS HARTMANN. 

